Deciding what purpose social media has for your brand is first priority, because the measurement follows from the goal. Most disagreements about social ROI are actually disagreements about purpose.
If the social goal is demand generation, track pipeline sourced and influenced by social using UTM parameters plus a self-reported attribution field on your forms, since last-click models undercount social severely.
If the goal for branded social is presence and credibility, track branded search volume, direct traffic, share of voice against named competitors, and whether win rates differ in deals where the prospect mentions your content.
If your goal is recruiting, track applicant volume and source quality.
